R¹ represents a methyl group;

R² represents a methyl group

R³ represents a methyl, ethyl or isopropyl group; and

-A- represents a ring which is either

(where R⁴ is a hydroxy or methoxy group and R⁵ is a hydrogen atom or R⁴ and R⁵ together with the carbon atom to which they are attached represent a group C=0); or when -A-represents ring (ii) wherein R⁴ and R⁵ together with the carbon atom to which they are attached represent a group C=0 then R² may also represent an aldehydo group;

or R² represents a hydroxymethyl group and -A- represents

(where R⁶ represents a hydrogen atom or a hydroxy group);

(where R⁷ is a hydroxy or methoxy group and Z represents either a group CHOH or a group C=0);

or R¹ represents a hydrogen atom and

(where R⁸ is a hydroxy or methoxy group); with the provisos that when R² represents a methyl group and R⁴ represents a methoxy group then R³ cannot represents a methyl or isopropyl group and when R² represents a hydroxymethyl group and R⁶ represents a hydroxy group then R³ cannot represent a methyl group.